Make CompositorObserver’s destructor protected-and-virtual.

ui/compositor/test/draw_waiter_for_test.h

 // Copyright 2014 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef UI_COMPOSITOR_TEST_DRAW_WAITER_H_
 #define UI_COMPOSITOR_TEST_DRAW_WAITER_H_
 
 #include <memory>
 
 #include "base/macros.h"
@@ skipping 20 matching lines @@
   // Waits for a commit instead of a draw.
   static void WaitForCommit(Compositor* compositor);
 
  private:
   enum WaitEvent {
     WAIT_FOR_COMMIT,
     WAIT_FOR_COMPOSITING_STARTED,
     WAIT_FOR_COMPOSITING_ENDED,
   };
   DrawWaiterForTest(WaitEvent wait_event);
-  ~DrawWaiterForTest();
+  ~DrawWaiterForTest() override;
 
   void WaitImpl(Compositor* compositor);
 
   // CompositorObserver implementation.
   void OnCompositingDidCommit(Compositor* compositor) override;
   void OnCompositingStarted(Compositor* compositor,
                             base::TimeTicks start_time) override;
   void OnCompositingEnded(Compositor* compositor) override;
   void OnCompositingLockStateChanged(Compositor* compositor) override;
   void OnCompositingShuttingDown(Compositor* compositor) override;
 
   std::unique_ptr<base::RunLoop> wait_run_loop_;
 
   WaitEvent wait_event_;
 
   DISALLOW_COPY_AND_ASSIGN(DrawWaiterForTest);
 };
 
 }  // namespace ui
 
 #endif  // UI_COMPOSITOR_TEST_DRAW_WAITER_H_